Eutrophication of lakes has become a global environmental problem. Harmful algal blooms (HABs) often result in serious problems, such as ecological function destruction, unpleasant odors, and water deoxygenation. In addition, it may be harmful to domestic animals and human.
At present, the main methods in controlling eutrophication of lakes and restoring the ecological environment include: (1) point and non-point pollution controlling to reduce the nutrients loaded to the lakes; (2) constructing wetlands; (3) bio-manipulation through feeding or capturing aquatic animals; (4) planting macrophytes; (5) controlling the water flow and level. The methods used in HABs and lake eutrophication control can be classified into three categories: physical methods, chemical methods and ecological methods. The physical methods include cutting off source of pollution, constructing retention basin, diluting, improving water level, circulating the water in lakes, dredging sediments, and collecting algae. However, these methods are all rather expensive. Chemical methods, including sewage dephosphorization, phosphorus-fixation, can often lead to secondary pollution. Ecological restoration has been widely studied for eutrophication control. For these methods, restoration of macrophytes is one of the most important methods to improve the water quality. Many methods have been made public as how to choose and combine the hydrophytes and how to plant these hydrophytes. However, the lack of clarity and transparency in HABs waters is the common limit for the successful restoration of submerged macrophytes. For natural waters, a transparency of 1.30 m˜73 m, 0.73 m˜40 m and 0.4 below Secchi disk transparence indicate eutrophication, moderate eutrophication and super-eutrophication, respectively. So, the first common problem to restore submerged macrophytes in the eutrophication lakes is the lack of methods that can effectively improve the water transparency at a very large scale, cost effectively, ecological safely, and high efficiently.
There are some hydrophytes restoration techniques in the eutrophication lakes, such as: planting vanguard hydrophytes and feeding zooplankton to improve the water clarity, constructing submerged macrophytes and emergent plant to improve the local water environment, adding oxidant (such as algaecide and fishacide) to improve the water quality. However, these methods need a long period to work effectively (usually several months or years), the manipulation is time-consuming and arduous, and the cost is very high. Moreover, these techniques can not be used in super-eutrophication water to restore hydrophytes. Therefore, to remove HABs and to reduce nutrient levels (e.g. total phosphorous) quickly, safely, cost-effectively and at a very large scale is the key limit for the successful restoration of submerged macrophytes.
The reported techniques in removing HABs or cyanobacterial blooms can be classified into four categories: chemical algaecides, using bacteria and virus to control algal growth, harvesting algal cells by floating and flocculation. Chemical algaecides are often forbidden to be used in many countries, mainly because of their adverse effects on other organisms. The cultured bacteria and virus control method is presently not an applicable option, mainly because of unsolved problems with its safety and handling. The floatation is almost inapplicable to large natural waters because of the high cost. The flocculation includes chemical flocculation and clay flocculation. The application of commercially available flocculants (such as polyaluminum chloride and polyferric chloride) to natural waters is often rejected due to economic reasons and safety concerns (such as the damage of bioecological balance and the dying of plankton). Nowadays, a very promising and environmentally friendly way to do this is to use natural, nontoxic and inexpensive clays to flocculate and remove algal cells (Anderson, D. M., 1997. Nature, 388, 513-514. However, clay methods still have many substantial problems, such as too high loading, sediment re-suspension and anaerobic release of pollutants/nutrients, and ecological safety concerns, that prevent the method to be extensively used in practical HABs control especially in fresh water systems.
In the early stage of the clay techniques, inorganic acid, hydrogen peroxide, surfactants, and polyunsaturated fatty acids have been used to modify clays and to improve the flocculation efficiency and to reduce the clay loading. However, these modification methods do not bring a breakthrough in reducing the loading and cost and in improving the safety. Gang PAN et al developed a series of techniques to improve the efficiency of removing algal blooms in fresh water lakes using clay flocculation. In 1999 and 2001, PAN developed a method for simultaneously clearing up HABs and harnessing organic pollutants to promote the primary productivity in the sea (UK patent, GB2337749) and a technique for flocculating and removing HABs using sepiolite (PRC patent, CN1417136) respectively. In 2002, PAN proposed a new modification technology by using natural netting and bridging polymer material, which can turn most soils or clays into highly efficient flocculants to remove algae cells in freshwater, and this technique made it possible to use local clays, soils and sediments to control local cyanobacterial blooms in lakes. Details about this technique can be seen in following references: 1. Pan, G., Zhang, M. et al. Environmental Pollution, 141 (2): 195-200; 2. Zou, H. Pan, G. et al, Environmental Pollution, 141 (2): 201-205; 3. Pan, G. et al, Environmental Pollution, 141 (2): 206-212.
Although the above-mentioned techniques of using local soils to flocculate and remove algal cells have greatly reduced the loadings and the costs of the materials, a common problem with all clay methods still exists. The flocculated algal cells which settle down to the sediment can decompose and consume a great deal of oxygen, which make the bottom waters/sediment become anaerobic. This will make some pollutants and nutrients be gradually released into the water, which will lead to the break out of HABs again. In order to solve this problem in 2003, PAN developed a technique using modified clays which can combine phosphorus fixation, algae flocculation and reversing of anaerobic environment in the sediment (CN200310113305.5). Furthermore, PAN developed a new technique using lanthanum to modify clays in 2005, which could make phosphorus in sediments effectively and nonreversiblely fixated in the modified clays (CN200510063410.1). By this way, even when sediments re-suspended, the probability of phosphorus in sediments releasing into the waters can be reduced. The two techniques mentioned above made further progress in controlling secondary pollution of sediments after removing algae. However, the above mentioned phosphorus-fixation method is still a physical chemical approach. It is difficult to prove the long term ecological safety of such kind of methods. The problem may be solved if physical chemical methods can be developed in combination with an ecological approach.
So far, there has been no method for algal removal and water quality improvement using modified local soils/clays/sediments that are simultaneously used for restoring submerged macrophytes in the technical field of ecological restoration. In addition, there has been no technology which combines physicochemical method and ecological restoration to control eutrophication in the technique field of algae removal using clays. Moreover, no technology has been reported to prevent algae resuspension, deoxygenation and secondary pollution using submerged macrophytes that are restored by using modified local soils/clays.